This invention relates to electrochemical machining (ECM).
ECM is a known technique for machining metal workpieces. A cathode is advanced towards the anodic workpiece in the presence of an electrolyte and a current is passed between the cathode and the workpiece through the electrolyte so as to cause material to be removed electrolytically from the surface of the workpiece.
This technique can be used for the machining of irregularly shaped workpieces such as dies and moulds, as well as irregularly shaped holes in metals which do not readily yield to mechanical cutting. Also, three-dimensional patterns can be applied to workpiece surfaces derived from a correspondingly shaped cathode.
High currents are desirable to attain high rates of removal of material, and the smaller the gap between the cathode and the workpiece the sharper is the machining definition which can be achieved.
However, with high currents and small gaps there is the problem that debris and any operational irregularities can give rise to adverse effects such as surface roughness, poor accuracy and even damaging short circuits. In practice therefore it is necessary to limit the gap to, say, no smaller than 0.2 mm, and this imposes a limitation on the sharpness of definition which can be achieved.
An object of the present invention is to provide an ECM technique with which very small gaps can be used with high machining quality, accuracy and productivity.
According to one aspect of the invention therefore there is provided an ECM technique wherein a cathode is advanced towards an anodic workpiece in the presence of an electrolyte and a current is passed between the cathode and the workpiece through the electrolyte so as to cause material to be removed electrolytically from the surface of the material characterised in that vibratory movement is imposed on the cathode so as to cause the gap between the cathode and the workpiece to vary, and the current is also varied.
With this technique it has been found that the vibration of the cathode and the variation of the current can counter adverse effects of debris and operational irregularities whereby it is feasible to use much smaller gaps and consequently sharper machining definition can be achieved. Gaps down to say 0.01 mm or even 0.005 mm may be feasible, compared with conventionally used gaps down to say 0.2 mm.
The vibratory movement applied to the cathode may comprise a main vibration preferably a periodic oscillation particularly of a low frequency, say in the range 1 to 100 Hz, conveniently of the order of 50 Hz. This oscillation may be a sine wave oscillation of constant characteristics and preferably it is applied wholly or largely along the direction of advancement of the cathode towards the workpiece.
With regard to the current variation, this may be of any suitable nature but preferably occurs on a periodic basis, which may be matched to, and preferably occurs at the same frequency as, the main vibratory movement of the cathode such that current peaks or pulses are delivered at or close to positions in the oscillatory cycle of the cathode corresponding to the smallest gap or nearest positioning of the cathode and workpiece.
Most preferably, the current variation has a fixed phase relationship with the main vibratory movement of the cathode such that the current pulses or peaks coincide with, or lag or lead to a predetermined extent, the smallest gap positions in the main vibratory movement cycle.
By arranging for current pulses or peaks to coincide with or be close to positions of maximum convergence between the cathode and workpiece erosion efficiency can be promoted. By arranging for the current to decline, or be switched off, as the cathode moves away from the workpiece it can be achieved that current flow is commutated thereby minimising stray erosion, which is adverse to accuracy. The period during which the gap increases and current flow decreases or terminates gives an opportunity for debris and machined particles to be flushed away.
Additionally or alternatively, the vibratory movement applied to the cathode may comprise a secondary vibration preferably of a higher frequency than the main vibration, generally of the nature of an ultrasonic oscillation, particularly having a frequency in the range 10 to 60 KHz i.e. 10 to 40 KHz or 20 to 60 KHz. This oscillation may be a sine wave oscillation or of any other suitable wave form
This higher frequency vibration can cause cavitation in the electrolyte between the cathode and the workpiece which dislodges debris and can allow operation with smaller gaps over larger areas without requiring unduly high current levels due to the blocking effect of bubbles. An even spread of electrolyte over the cathode and workplace surface can be facilitated. Also the cavitation can help remove metal oxide film and thereby facilitate activation of machining on oxidised metals.
Most preferably this secondary vibratory movement is applied to the cathode simultaneously with the aforesaid main vibratory movement.
Preferably also, the secondary vibration is applied wholly or largely along the direction of advancement of the cathode towards the workpiece.
The secondary vibratory movement of the cathode may occur continuously with constant, regular characteristics. Alternatively, the vibration, may be discontinuous, and/or may vary or be irregular with regard to frequency, amplitude, mark-space ratio or any other characteristic as desired. Thus, by way of example, the secondary vibration can be frequency and/or amplitude modulated and can be applied as individual pulses or as packages of pulses and may be locked to the variation (e.g. frequency) of the electric current and/or to the frequency of the main vibration.
In a preferred embodiment the secondary vibration movement is tuned in relation to the cathode""s mechanical properties to give resonance.
A control system is preferably provided to effect automatic control of machining parameters, and conveniently this system may be computerised.
Thus the control system may control advancement of the cathode as material is removed from the workpiece surface so as to maintain a desired cathode/workpiece gap. This may be achieved by monitoring current and/or voltage characteristics across the gap. Additionally or alternatively other indications may be utilised such as optical or acoustic monitoring of the gap. In the latter respect, where ultrasonic secondary vibratory movement is applied to the cathode as mentioned above this can result in the generation of an acoustic signal dependent on the magnitude of the gap and this can be monitored with a transducer.
The control system may also control advancement in relation to a determined starting reference position so as to achieve a desired depth of machining in the workpiece. This reference position may be established by determining the position of the cathode when the workpiece is contacted by the cathode, preferably at a bottom-dead-centre position of vibratory movement of the cathode.
The control system may operate to control advancement of the cathode so as to maintain constant parameters for the gap. Alternatively however the control system may operate to vary the gap depending on factors such as detected variations in machining conditions, or the stage in the machining process e.g. such that initial machining takes place with a larger gap and final precision finishing takes place with a smaller gap.
Alternatively or additionally the control system may control voltage and/or current across the cathode/workpiece gap so as to maintain a desired rate of machining, which may be a constant rate or a varying rate. In the latter case, the machining rate may be varied in dependence on machining conditions and/or stage in the machining process.
Provision may be made for pre-setting the control system in accordance with different requirements, relating for example to different materials, or different types or characteristics of shapes to be machined. Provision may also be made for pre-setting other parameters for this purpose, in particular, parameters of the main and/or secondary vibration and/or the current variation, as mentioned above.
The control system may also be utilised to monitor and maintain at a predetermined or pre-set value parameters relating to the supply of electrolyte, particularly the pressure of the electrolyte.
The electrolyte is preferably caused to flow between the cathode and workpiece e.g. by pumping from an inlet to an outlet through a vessel or shroud enclosing at least parts of the cathode and workpiece.
The electrolyte may be supplemented by an injected aqueous medium which may contain an acid or alkali and/or a salt solution and/or abrasive particles.
The invention also provides a machine for use in performing the method described above comprising a cathode support, a workpiece support, means for supplying electrolyte between the cathode and workpiece, means for supplying current to the cathode and the workpiece, means for advancing the cathode towards the workpiece, means for applying vibratory movement to the cathode to vary the gap between the cathode and the workpiece, and means for varying the current supplied to the cathode and the workpiece.
In addition the advancement and vibratory movement of the cathode, provision may also be made for other movements to facilitate machining of different or larger shapes. Thus, provision may be made for movement in one or more axes transversely to the direction of advancement and/or rotation of the cathode about the direction of advancement.